The present invention is related to a method for reducing the metal contamination on a surface of a semiconductor substrate which is submitted to a wet cleaning process.
One of the challenges in semiconductor device processing is the problem of metal contamination on the semiconductor surface. Semiconductor devices are extremely sensitive to minute quantities of metals adsorbed to the semiconductor surface. These metallic species are well known to deteriorate electrical performance. Particularly, Group IIA metals have been found to be very detrimental to the gate oxide layers. There is an important need to reduce metallic contamination to low levels.
The RCA cleaning sequence which is still standard in semiconductor (silicon wafer) device processing was devised in the years 1970s. The SC1 or Standard Clean 1 process, using a mixture of concentrated NH3 and H2O2 mixed with H2O in a ratio of 1:1:5, is used largely to remove particulates. The SC2 or Standard Clean 2 process, using a mixture of HCl, H2O2 and H2O, is used to remove metallic contaminants from the silicon surface. These SC1 and SC2 cleaning steps, in either order are each followed by a rinsing step in DI water, whose purpose is to remove acidic or caustic species from the wafer and to remove metallic contaminants remaining in solution near the wafer surface from proximity with the wafer.
Current cleaning and rinsing techniques have proven to be unsuccessful at reducing metallic contaminants to the low levels required without significantly increasing at great the cost of the DI rinse water supply purity. The final rinse of the cleaning process contributes to metal contamination, even in ultra pure water (UPW), and must be improved. The microelectronics industry also needs to lower its costs. In cleaning, this can be done through the introduction of simplified cleaning recipes, including a single chemistry clean that has both good particle and metal performance. Ammonia-peroxide-water mixtures (APM) today have good particle performance, but their performance with respect to metallic contamination needs much improvement.
It was shown by Hall and co-workers (Hall et al., IEST 1998 Proceedings, the ICCCS 14th Int. Symp. On Contamination Control, Apr. 26-May 1, 1998) that calcium contamination and contamination by other metals is worse for dilute SC1 chemistries, where the concentrated chemicals NH3, H2O2 and H2O are mixed in a ratio of 1:1:30, than for SC1 solution, where the concentrated chemicals NH3, H2O2 and H2O are mixed in a ratio of 1:1:5. Hall""s explanation for the reduced adsorption of Ca and other metal ions is that they form amine complexes, that is M(NH3)n+, where n typically is between 1 and 6.
The Applicants have previously suggested that H+ always is present in the environment of the silicon wafer so that metals do not attach to the silicon substrate (see EP-96309145.9 incorporated herein by reference). This is indeed a possible solution to the recontamination problem, but it is not applicable in all circumstances. Notably, it is not possible to keep the wafer environment always acidic during a rinse or during APM or SC1 treatment. Coming out of an SC1 process or a similar process, wafers would go through various levels of alkaline solution, and then pass through neutrality, before reaching an acid rinse condition, even if an acidic rinse is carried out.
U.S. Pat. No. 5,290,367 discloses a surface cleaning solution for use in semiconductor processing, said solution containing as a complexing agent a compound having one or more phosphoric acid groups or a salt thereof and a polyphosphoric acid or a salt thereof. The complexing agent is binding to metals and therefore reduces the adsorption of metals to the semiconductor surfaces.
U.S. Pat. No. 5,466,389 discloses an alkaline cleaning solution for use in semiconductor IC processing. The invention is aimed to avoid the presence of unstable compounds such as H2O2 in cleaning solutions used for semiconductor processing. The invention furthermore aims to achieve semiconductor wafer cleaning with such cleaning solutions without causing undue wafer roughness.
The compound N(CH3)4OH is known to be a possible alternative for NH4OH in SC1 solutions. N(CH3)4OH (TMAH) is strongly dissociating in aqueous environments.
Other prior art patents disclosing methods for reducing the adherence of metallic impurities to semiconductor surfaces include EP649168 and EP528053. The solutions as disclosed in these patents are successful in reducing the contamination level of only a limited number of metal ions. The addition of phosphonic acid to the cleaning solution disclosed in EP528053 can introduce undesired organic residue on the substrates and the phospor element may lead to an undesired doping of the semiconductor substrate. EP649168 furthermore is limited to acid solutions having a pH of lower than about 5.
The present invention includes a method for enhancing the metal removal capability of a semiconductor substrate in a substrate cleaning process, without requiring greatly increased chemical purities.
By xe2x80x9cenhancing the metal removal capabilityxe2x80x9d, it is to be understood for the purpose of the present patent application, reducing surface concentrations on the surface i.e. removing the metal contaminants from the surface of the semiconductor substrate and/or preventing the adsorption of metal contaminants present in the chemical solution onto said surface.
The present invention therefore aims to enable a cost-effective single chemistry cleaning.
The present invention is related to a method for reducing the metal contamination on a surface of a semiconductor substrate wherein said substrate is submitted to a wet cleaning process in a solution capable of oxidising said surface and containing a substance for substantially reducing the amount of metal ions bound to the oxidised surface.
In a first aspect of the present invention, a method for reducing the metal contamination on a surface of a semiconductor substrate is disclosed. According to the method of the invention, said substrate is submitted to a wet cleaning and/or rinsing process in a solution capable of oxidising said surface and containing a substance strongly ionising in said solution whereby creating an amount of ions of at least one species in said solution. At least one of the ion species is having such size and shape that the ions of this species are binding strongly to the oxidised surface and said amount of ions is sufficient to substantially reduce the amount of metal ions bound to the oxidised surface.
The substance is strongly ionising in said solution whereby creating an amount of ions of at least one species in said solution. Thus the substance is dissociating, or possibly hydrolysing, into ions. The ions preferably are plurality NH4+ ions and/or derivates and/or compounds thereof that have the beneficial impact on reducing the amount of metal ions bound to the oxidised surface. With the term strongly ionising, it is meant that at least about 6% of the molecules of the substance is dissociating into ions. Under the term strongly ionising, it is also understood that about 7 to 8 or 9-10-15-20-30-40-50-60-70-80-90-100% of the molecules of the substance are dissociating into ions. These numbers of strong ionisation are given for room temperature conditions, equivalent numbers being relevant for higher or lower temperature conditions of practising the invention. The invention can be practised at temperatures lower (above 0xc2x0 C.) or higher (for instance 50 or 60 or 80 or 100xc2x0 C. or higher) than room temperature. The invention can further be practised at ambient temperature.
Preferably the solution is at a pH value of about 5 or at a pH value of about 5.5 or 6 or 6.5 or 7 or higher. Preferably also said solution does not contain in combination a nonionic surfactant and an effective amount of a pH reducing chemical component to reduce or control the pH of the solution to a pH within the range of from about pH 8 to about pH 10. The combination of chemical components in the solution in the latter combination would lead to a competing mechanism of surfactants absorbing on the surface of the substrate. The teaching of the patents U.S. Pat. No. 5,466,389 and EP0678571 are both incorporated herein by reference in their entirety in order to illustrate the effect of the latter combination of components.
In a preferred embodiment of the invention, the amount of ions of the species is present in such concentration that the concentration of said metal ions on said surface is reduced to below 1012 or 1011 or even 1010 atoms/cm2. At least one of the ion species is such that the ions of the species are binding to the oxidised surface in such a way that said amount of ions is substantially reducing the amount of metal ions bound to the oxidised surface. Thus the ions of this species have such size and shape that the ions (in the specification also referred to as cations or benign cations) are binding strongly to the oxidised surface and can remove metal ions bound to the oxidised wafer surface. The property of one cation""s ability to suppress the adsorption of metal ions is part of the invention. Under all circumstances, whether in acidic, alkaline or neutral solution, metal ion adsorption to the wafer surface is reduced by the addition of cations to the cleaning solution. In acidic solution, H+ may be sufficient to reduce metal adsorption, but the addition of cations such as NH4+ or other cations can significantly suppress metal adsorption.
Said substance can be present in a concentration larger than about 10xe2x88x925 or 10xe2x88x924 or 10xe2x88x922 mol/l.
Said solution preferably comprises ammonia. Said solution can further comprise ammonia, hydrogen peroxide and water in a relation of about 1:1:5. It is to be understood for the purpose of this application that xe2x80x9chydrogen peroxidexe2x80x9d referred to in the :: signs is a solution of about 28% of H2O2 in H2O. It is to be understood for the purpose of this application that xe2x80x9cammoniaxe2x80x9d referred to in the :: signs is a solution of about 30% of ammonia in H2O.
Said solution can also comprise a mixture of ammonia, hydrogen peroxide and an alcohol. The alcohol may be selected from the group consisting of isopropanol, ethanol and mixtures of isopropanol and ethanol.
In an embodiment of the first aspect of the invention, the method can comprise the steps of:
preparing said solution capable of chemically oxidising said surface and containing said substance; and thereafter
immersing said substrate in said solution while maintaining processing conditions such that said substance is substantially reducing metal ions of binding to the oxidised surface.
The method can also comprise the steps of:
exposing said surface to an aqueous solution containing an oxidising agent and a base agent while maintaining processing conditions such that a chemical oxide is formed on said surface; and
adding to said solution said substance capable of binding to the oxidised surface while maintaining processing conditions such that said substance is substantially reducing the amount of metal ions bound to the oxidised surface.
Said solution preferably has a pH of about 7, a pH of 7.5 or a pH of 8 or of 9 or even higher (e.g. of 10).
According to second aspect of the present invention, a chemical solution for use in semiconductor device processing is disclosed, said solution consisting essentially of water, an oxidising agent, a base agent and a salt of ammonium.
A third aspect of the invention is to disclose the use of a chemical solution comprising water, an oxidising agent, a base agent and a salt of ammonium for cleaning and/or rinsing of semiconductor substrates, the amount of the salt of ammonium in said solution being such that the presence of metallic contaminants on the substrates is reduced.
According to another aspect of the present invention, a chemical solution for use in semiconductor device processing is provided. The solution consists essentially of water, an oxidising agent, a base agent and a salt of ammonium. In one embodiment, the salt in the solution is selected from the group consisting of NH4F, NH4Cl, NH4Br, NH4I, NH4NO3, and (NH4)2SO4.
According to still another aspect of the present invention, a chemical solution for use in semiconductor device processing is provided. The solution consists essentially of an alcohol, an oxidising agent, a base agent and a salt of ammonium, the alcohol selected from the group consisting of isopropanol, ethanol, and mixtures of ethanol and isopropanol. In one embodiment, the salt in the solution is selected from the group consisting of NH4F, NH4Cl, NH4Br, NH4I, NH4NO3, and (NH4)2SO4.
Yet another aspect of the invention is to disclose a method of using a chemical solution with semiconductor substrates. The chemical solution comprises a liquid, an oxidising agent, a base agent and a salt of ammonium for cleaning and/or rinsing of semiconductor substrates, the liquid being selected from the group consisting of water, isopropanol, ethanol, and mixtures of ethanol and isopropanol, the amount of the salt of ammonium in said solution being such that the presence of metallic contaminants on the substrates is reduced.
Still another aspect of the invention is to disclose a method of producing a chemical solution for use in a cleaning and/or rinsing step of semiconductor device processing. The solution comprises a liquid and an oxidising agent, the liquid being selected from the group consisting of water, isopropanol, ethanol, and mixtures of ethanol and isopropanol, the method comprising the step of admixing an acid agent and a base agent in said solution while at least one semiconductor substrate is cleaned in said solution.
Yet another aspect of the invention is a method of producing a chemical solution for use in a cleaning step of semiconductor device processing. The solution comprises a liquid and an oxidising agent, the liquid being selected from the group consisting of water, isopropanol, ethanol, and mixtures of ethanol and isopropanol, the method comprising the step of admixing an acid agent and a base agent in said solution prior to exposing one semiconductor substrate to said solution. The acid agent, in one embodiment, is selected from the group consisting of HCl, HBr and HNO3 and said base agent is NH3.
In the different aspects of said semiconductor substrate is one of a batch of wafers being simultaneously processed in a single tank, tanks, or wet bench or a single wafer in a single wafer cleaning tool.